In the United States, there are over one thousand drinking water treatment plants which use alum, Al2(SO4)3.14H2O, as a coagulant for efficient removal of particulate solids and colloids. In the treatment process, alum is finally converted into insoluble aluminum hydroxide, Al(OH)3, which constitutes a major component, e.g., from about 25% to 50%, of the solids in water treatment residuals (WTR), i.e., clarifier sludge. The water treatment sludge is essentially a bulky, gelatinous slurry composed of suspended inorganic particles, natural organic matter (NOM), trace amounts of heavy metal precipitates and aluminum hydroxide. Clarifier sludges are biologically inert and retain near-neutral pH. The total solids content of the sludge normally ranges from 2 to 10 percent in mass per unit volume. Water treatment plants in the United States produce over 2 million tons of aluminum-laden disposable solids every day. Due to recent regulatory changes, disposal of sludges must now be carried out by way of landfills or land application. Because of the magnitude and pervasiveness of the disposal problem, alum recovery from clarifier sludge has received considerable attention. The toxicity of free and complexed aluminum species to aquatic life, including benthic organisms, is also a matter of concern, and has been the focus of several studies.
An ideal solution to the problems of sludge disposal and toxicity would be a simple-to-operate process which selectively recovers alum from the sludge to reduce the volume of the disposable solids, and which delivers the recovered alum in a form sufficiently pure to be recycled for use as a coagulant at the front end of the water treatment plant. Such a process will truly combine pollution prevention with resource recovery, thereby significantly reducing the stress on the environment.
When clarifier sludge is sufficiently acidified with sulfuric acid, insoluble aluminum hydroxide is dissolved in the form of dilute liquid alum. The stoichiometry of this reaction is as follows: 
This reaction illustrates the underlying concept of the acid digestion process, which has been tried both at laboratory and pilot-scale levels.
The basic concept of the process of alum recovery is simple. However, the process is subject to shortcomings which have ruled out the possibility of reusing the recovered liquid alum as a coagulant. The process is non-selective. Along with alum it also recovers all other substances that are soluble under highly acidic conditions or that exist as colloids. If this occurs and the recovered alum is recycled for water treatment, the potability of the water will be degraded. Consequently, the acid digestion process may not be used in areas where such impurities present problems.
Potential impurities which can be converted to soluble form by acidification include iron, manganese, chromium and other metals, including those metals which are inherently present as impurities in the sulfuric acid used in the process. For example, significant concentrations of manganese exist in raw water at some locations, and the concentration of manganese in the delivered water may be increased to unacceptable levels when alum is recovered for reuse as a coagulant. Heavy metals, such as copper, lead, cadmium, etc. are normally present in relatively low concentrations in clarifier sludge. However, the concentrations of these heavy metals may increase to significant levels if the sludge is recycled in order to recover alum. The problem of increased concentration of an undesirable substance in the recovered alum can occur with many different substances present in ionic or colloidal form in the raw water, especially substances which have a low solubility and a high settling rate in the clarifier.
Naturally occurring organic material (humates and fulvates), which are generally removed quite well by alum coagulation, will be present in the recovered alum. Should this recovered alum be reused as a coagulant, the concentration of organic matter in the treated water will tend to increase, thereby significantly increasing the potential for formation of trihalomethanes (THMs), which are suspected carcinogens.
Since aluminum oxide is amphoteric, theoretically alum could be recovered from clarifier sludge under alkaline conditions as well as under acidic conditions. However, dissolved organic carbon (DOC) tends to increase with dissolved Al(III) under both acidic and alkaline conditions. Since a high concentration of dissolved organic matter is very undesirable in recovered alum due to its potential for formation of trihalomethanes, neither acid nor alkali digestion processes have been able to achieve satisfactory selective alum recovery in practice.
Acid digestion, that is, acidic extraction using sulfuric acid, is the most widely used method for alum recovery. When clarifier sludge is sufficiently acidified with sulfuric acid, insoluble aluminum hydroxide is dissolved in the form of dilute liquid alum as shown in the equation:
2Al(OH)3.3H2O+3H2SO4+2H2O=Al2(SO4)3.14H2O
This equation illustrates the underlying concept of the acid digestion process, which has been tried on a laboratory scale, in pilot scale studies, and in full-scale at one of the water treatment plants of Durham, N.C. Studies on this process have shown that the aluminum concentration in recovered supernatant liquid ranged from 360 to 3700 mg/l. In addition to aluminum, the recovered alum was also found to contain other metals such as manganese, zinc and lead. The concentration of heavy metals such as As, Cr, Cu, Ni, Pb and Zn ranged from 0.002 to 8.5 mg/l. The total DOC ranged from 326 to 1800 mg/l, which was of the same order of magnitude as the recovered aluminum concentration. Similarly, the concentration of humic substances ranged from 160 mg/l to 1140 mg/l. Thus, the acid digestion process is non-selective; it cannot prevent DOC and heavy metals from being carried over into the recovered alum. The process is not capable of yielding high concentrations of aluminum ions, and the recovered alum is subject to reduced coagulation efficiency due to the presence of dissolved organic carbon.
In the case of Alkali digestion, at increased pH, the total DOC increases markedly. At a pH of 12, the DOC of water treatment residuals increases rapidly to over 1000 mg/liter. Furthermore at high pH levels, undissolved organics tend not to settle adequately and lead to poor quality in the recovered solution.
Liquid Ion Exchange (LIE) has also been tried. According to one study, this process can concentrate aluminum to a level as high as 4000 mg/l from an initial level of 1000 mg/l. However, this takes place in a second stage of stripping. In the first stage of extraction, the concentration ratio is 1:1. Entrainment issues are always a concern in the stripping process, because it involves separating aluminum ions from an organic phase into which the aluminum ions are dissolved in the first stage. In the stripping process, aluminum is recovered from the organic phase and the latter recycled. Since ideal 100% separation cannot be achieved, organics are carried over with dissolved aluminum. The liquid ion exchange process is operationally complex and expensive, and capable of delivering only a low concentration of recovered alum. Solvent carryover in the recovered alum is also a problem inherent in liquid ion exchange, and requires additional treatment steps.
Ultrafiltration is another technique that can be employed following acid treatment. However, ultrafiltration suffers from various shortcomings including fouling, decreased membrane life due to pressure differential, a decrease in flux with continued deposition, and the relatively high cost of pumping.
Still another approach is the cyclic composite membrane process described in U.S. Pat. No. 5,304,309, dated Apr. 19, 1994. In this process, aluminum ions are selectively sorbed from an aqueous phase (containing dissolved aluminum in acidified WTR) onto a composite membrane and thereafter desorbed, with the release of aluminum ions, as the composite membrane is regenerated in a sulfuric acid solution. The process is carried out in a continuous cycle characterized by the following equations:
xe2x80x83Al(OH)3(s)+3/2Rxe2x80x94N(CH2COOH)2=3/2Rxe2x80x94N(CH2COOxe2x88x92)2Al3++3/2H2O
3Rxe2x80x94N(CH2COOxe2x88x92)2Al3++3H2SO4=3/2Rxe2x80x94N(CH2COOH)2+Al2(SO4)3
The cyclic composite membrane process overcomes many of the shortcomings of the previous processes in that it selectively recovers aluminum ions and prevents passage of natural organic materials, heavy metals and manganese into the recovered alum. However, composite ion exchange materials are not available in sizes appropriate for large-scale applications, and the process is not capable of concentrating alum to high levels. Furthermore, the process is a two-stage process with inherent complexity, and subject to various operational problems such as difficulties in rinsing the membrane.
An object of this invention is to overcome at least some, and preferably all, of the above-mentioned deficiencies in previously proposed alum recovery techniques.
In accordance with the invention, the pH of clarifier sludge is adjusted to a level, preferably about 3, such that at least the majority of its aluminum content is in solution. The pH adjustment results in an aqueous clarifier sludge solution, which is then brought into contact with a first side of a semi-permeable cation exchange membrane, preferably by circulating the solution through a first circulatory flow path. Simultaneously, an acidic sweep solution is brought into contact with the opposite side of the membrane, preferably by circulating it through a second circulatory flow path. By Donnan dialysis, aluminum ions are caused to pass through the membrane in one direction while hydrogen ions pass through the membrane in the opposite direction. Anions, organic molecules and suspended solids are substantially prevented from passing through the membrane, and aluminum ions are selectively favored over monovalent and bivalent cations for passage through the membrane. Thus, the concentration aluminum in the second circulatory path increases while the solution in the second circulatory path remains substantially free of anions and organic molecules, the concentration of toxic metals in the second circulatory flow path is maintained at a low level, and the hydrogen ions regenerate the membrane and acidify the aqueous clarifier sludge solution thereby causing further quantities of aluminum in the first circulatory path to go into solution.
In a preferred mode, the cation exchange membrane comprises a plurality of sheets of cation exchange membrane disposed in a stack, the sheets being separated from one another in the stack by spaces through which the solutions flow, and the solutions are directed to and from the stack so that clarifier sludge solution and acidic sweep solution flow through alternate spaces. Preferably the spaces through which the clarifier sludge solution flows are connected in series, and the spaces through which the acidic sweep solution flows are also connected in series.
The process is preferably carried out with the pressure difference across the membrane maintained substantially at zero.
The process provides for a simple and effective recovery of alum, substantially free of solids, natural organic matter and dissolved organic carbon, and with relatively low carryover of toxic metals. The recovered alum is therefore suitable for reuse as a coagulant in the same water treatment plant from which it was recovered, and the formation of trihalomethanes upon chlorination is substantially avoided.
Further objects, advantages and details of the invention will be apparent from the following detailed description when read in conjunction with the drawings.